Field of the Invention
The present invention relates to a method for controlling the injection through an injection molding machine, and more specifically to a method for controlling the injection through an in-line screw type injection molding machine equipped with a check ring, i.e., a check valve for preventing molten resin from flowing backward from a side of a free end portion of a screw to a rear side of the screw during an injection stroke.
As has been known in the art and is illustrated in FIG. 8, an in-line screw type injection molding machine is designed to knead and plasticize a resin material, which has been fed from an unillustrated hopper, by the rotation of a screw 2 in a normal direction, namely, in the direction indicated by an arrow A within a heating cylinder 1 and then to store the resultant molten resin 3 on the side of a check head 4 mounted on the free end portion of the screw 2 under the feeding action of a thread groove of the screw 2. At this time, a check ring 5 functioning as a check valve toward the rear of the check head 4 moves to its front-most position due to the pressure of the resin fed by the screw 2, so that the check ring 5 is away from its check seat 6. The molten resin 3 is therefore fed forward through a spacing between the check ring 5 and the check seat 6, another spacing between an inner peripheral wall of the check ring 5 and a neck portion 4a of the check head 4, and grooves 4d in a head portion 4b of the check head 4. As the molten resin 3 is stored on the side of a free end portion of the check head 4, the screW 2 retracts in the direction indicated by an arrow C. When the screw 2 has retracted over a predetermined distance, in other words, when sufficient molten resin 3 for the injection shot has been stored (namely, at the metering completion point P.sub.1 in FIG. 9), rotation of the screw 2 ceases, enabling completion of a mixing and kneading, plasticizing and metering stroke.
When an injection command is outputted after the above mixing and kneading, plasticizing and metering stroke, the screw 2 advances in the direction indicated by an arrow D (see FIG. 9) so that the molten resin 3 stored adjacent to the free end portion of the check head 4 is injected into the cavity of an unillustrated mold through a nozzle 7 fitted in a free end of the heating cylinder 1. At this time, the pressure build-up in the molten resin 3 stored adjacent to the free end portion of the check head 4 causes the check ring 5 to retract and an annular end face 5a of the check ring 5 is thus brought into close contact with check seat 6. As a result, the molten resin 3 stored adjacent to the free end portion of the check head 4 is prevented from flowing back to the rear side of the screw 2.
During the above mixing and kneading, plasticizing and metering stroke, the molten resin 3 stored adjacent to the free end of the check head 4 is prevented from dribbling out through the nozzle 7 by the resin which has been injected into the mold. When the mold is opened subsequent to the mixing and kneading, plasticizing and metering stroke, the molten resin 3 tends to undergo drooling through the nozzle 7 fitted in the free end of the heating cylinder 1 because the pressure of the molten resin 3 stored adjacent to the free end portion of the check head 4 is substantial. This drooling is known to cause problems such as silver blisters.
To avoid this problem, it has been conventional practice that, as shown in FIG. 10, the screw 2 is forced to retract further by about several millimeters from the aforementioned metering completion point P.sub.1 to a point P.sub.2 after the completion of the mixing and kneading, plasticizing and metering stroke but before the injection stroke, whereby the pressure of the molten resin 3 stored adjacent to the free end portion of the check head 4 is reduced. This procedure is called "decompression control" and is effective for the prevention of the above-described drooling.
However, the decompression control involves the problem that the molten resin 3 metered and stored adjacent to the front end portion of the check head 4 and the molten resin 3 fed to and stored on the rear side of the check head 4 are communicated to each other and are allowed to flow and the quantity of the former molten resin 3 already metered and stored is thus changed. This problem occurs in the following manner. Since the molten resin 3 has the same pressure on both the front and rear sides of the check ring 5 after the completion of the mixing and kneading, plasticizing and metering stroke, the above-mentioned retract of the screw 2 by the decompression control results in a lower pressure on molten resin 3 metered and stored adjacent to the free end portion of the check head 4 than that on the molten resin 3 fed to and stored on the rear side of the check ring 5. As a result, as depicted in FIG. 10, the check ring 5 is caused to separate from the check seat 6 and fails to function as a check valve. The quantity of the molten resin 3 stored adjacent to the free end portion of the check head 4 has therefore been changed. This has led to the problem that molded products vary fractionally from one shot to another due to minute variations in the corresponding metered quantities of the molten resin.